Siemens SIMATIC S5 Manual page 254

Access commands for the S5-115U, S5-135U and S5-155U programmable controllers
Word- by-word
access to
consistent data
area
Byte-by-byte
access to
consistent data
area
B-14
The following rules apply to word-by-word access to consistent areas:
If you access these modules only through the process image (PII, PIO),
the data is always consistent.
If you use load/transfer commands for direct access to consistent areas,
observe the following access rules:
– Access consistent data only word-by-word (address must be an even
number)
– Always read or write the job number or control word (CPs and IPs)
last, in other words read the parameters first and then the job
number.
This means you should always start by accessing the word which
contains the switch-off byte.
The following rules apply to byte-by-byte access to consistent areas:
If you access these modules only through the process image (PII, PIO),
the data is always consistent.
If you access consistent areas directly with load/transfer commands, you
must always access the switch-off byte last. The correct switch-off bytes
are listed in Sections B.6.1 to B.6.6 and depend on:
– CPU type
– type of consistency
– address area (0 to 127 in P area or addresses outside this area)
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a